URBAN TREES AS A RAINWATER MANAGEMENT TOOL: “We found that individually planted trees capture, store and release stormwater back into the atmosphere—a process called transpiration—at a rate three times that of trees in a forest,” stated doctoral candidate Sarah Ponte, Department of Environmental Science and Technology at the University of Maryland (November 2021)

Note to Reader:

“It’s hard to overstate the environmental importance of trees, which among other functions pull climate change-inducing carbon from the atmosphere, clean the air of toxins and help control runoff,” stated a news release by the University of Maryland.

“While it can likewise be hard to quantify some of these effects, a new study by University of Maryland researchers helps clarify the role of urban trees in mitigating stormwater flows, and finds that even isolated trees lining a street or planted in a park may have a significant effect.”

Urban Trees are a Singular Weapon in Rainwater Runoff Management

A study published in the journal Scientific Reports by Assistant Professor Mitch Pavao-Zuckerman and doctoral candidate Sarah Ponte, both of the Department of Environmental Science and Technology at the University of Maryland, explores how trees function in different urban contexts, from streets to small patches of forest.

The University of Maryland team measured transpiration in three distinct urban settings: single trees over turfgrass and a cluster of trees over turfgrass in Montgomery County, and a closed canopy forest in Baltimore. They built and used sap flux sensors to obtain a clearer picture of how trees access groundwater, installing them in 18 mature red maple trees to continually monitor transpiration rates during the growing season. They also measured soil water content, air temperature, relative humidity and precipitation at each site.

Application of Research Findings

This is knowledge that can help support the management of green infrastructure, including the rising practice of tracking the environmental service provided by trees to calculate rainwater runoff (stormwater) or other fees assessed by municipalities.

“Understanding how different management contexts affect urban ecohydrologic fluxes, such as transpiration, can aid the development of policy on the application and effectiveness of urban tree canopy as a tool for stormwater runoff reduction at watershed and city scales,” stated Sarah Ponte.

“Our data can help make tree-crediting policies better reflect the actual benefits of trees in urban landscapes, because they interact with water and their environment differently in cities than they do outside cities,” added Mitch Pavao-Zuckerman. “Our next step is to take this data set on how each tree functions and scale it up to see how an entire stand or patch of trees mitigates stormwater flows.”

Guidance for Achieving the Greatest  Benefit

“Quantifying the impacts of urban trees affect different parts of the water balance, such as the evapotranspiration component discussed in Mitch and Sarah’s paper, gives us a better understanding of the benefits of urban trees, and knowing where and how to plant and preserve them to achieve the greatest benefit.” said Deb Caraco, senior watershed engineer with the Center for Watershed Protection.

To Learn More:

Download a copy of the paper by Sarah Ponte et al titled Transpiration rates of red maple (Acer rubrum L.) difer between management contexts in urban forests of Maryland, USA.


Tree Canopy Climate Station installation in Clovelly-Caulfeild neighbourhood, West Vancouver

Tree Canopy Interception Research Project in the North Shore Region of Metro Vancouver

“It was with great interest that I read about the research that Sarah Ponte and Mitch Pavao-Zuckerman have been doing at the University of Maryland. It brought back memories of our bold initiative in British Columbia almost two decades ago,” states Richard Boase, Section Manager Environmental Sustainability (Operations) at the District of North Vancouver. Richard is also a founding Director and Vice-President of the Partnership for Water Sustainability in British Columbia.

“Trees have long been a career focus for me because of my role at the District of North Vancouver role. This led to my involvement in the development of the Water Balance Model for British Columbia as a decision support tool to help communities make better informed land use decisions. During the period 2005 through 2010, the Partnership undertook and I chaired the North Shore Tree Canopy Interception Research Project. This was a precedent-setting project, both in terms of the research scope and the coalition of funders.”

Dr. Markus Weiler elaborates on the science at the project initiation meeting in September 2005.

Project Initiation in 2005

“It was exciting how quickly this project came together. Within 12 months of our initial exploratory meeting with Hans Schreier and Markus Weiler, we were underway. We made it happen thanks in large part to Hans Schreier, our champion at UBC. He brought Markus Weiler into the fold because he recognized the potential synergy between academia and local government in an area of common interest. In 2005, all of us could see the mutual benefits and strategic importance of fostering a long-term relationship between the University and the Partnership.”

“The Province of British Columbia, Metro Vancouver Regional District, Real Estate Foundation of British Columbia, and three North Shore municipalities funded the project start-up. When the University of British Columbia (through the visionary efforts of Dr. Hans Schreier) obtained a research grant from the Canadian Water Network, this brought a national focus to the project. We then installed a network of 60 tree canopy climate stations across the North Shore region (District of North Vancouver, City of North Vancouver, and District of North Vancouver).”

To Learn More:

Explore the historical resources accessible at North Shore Tree Canopy Interception Research Project

Dr. Markus Weiler, Research Team Leader

“Dr. Markus Weiler, Chair of Forest Hydrology at UBC, led the research effort. Markus was hands-on. His enthusiasm, creativity, and hands-on involvement were difference-makers in getting such a bold project implemented,” recalls Richard Boase. “However, Markus returned to his native Germany within a few short years and he was never adequately replaced. We have often wondered how far ahead we would be today if Markus Weiler had remained in British Columbia to provide leadership and be the champion for further university research into the rainwater interception benefits of the urban tree canopy.”

“While considerable research has been undertaken in forest stands in the natural environment, very little had been done (as of 2005) in an urban setting anywhere in North America,” reported Markus Weiler at the time of research project initiation. “As I see it, this initial collaboration between UBC and the Partnership opens the door to a long-term partnership to bring science into the community. Because of the urban context for the proposed research, our focus will be on quantifying the interception effectiveness of a single tree versus that for a cluster of trees.”

“The limited research that had been done in North America was undertaken by the Center for Urban Forest Research at the University of California Davis,” notes Richard Boase. “The Partnership had collaborated with the Center during initial development of the Tree Canopy Module for the Water Balance Model.”

To Learn More:

Download a presentation by Markus Weiler titled Tree Interception and Runoff Generation, June 2007

Tree Canopy Research Project engaged Clovelly-Caulfeild Neighbourhood in West Vancouver

Collaboration between researchers at the University of British Columbia and the Metro Vancouver region’s three North Shore municipalities — North Vancouver District, North Vancouver City, and the District of West Vancouver — opened the door to bringing science into the community. Clovelly-Caulfeild in West Vancouver was the first North Shore neighbourhood to step forward and participate in the North Shore Tree Canopy Interception Research Project. Clovelly-Caulfeild is situated in the western end of the municipality.

An inter-municipal coordinating team developed tree/site selection criteria and, with the assistance of Dr. Markus Weiler, developed an innovative system for capturing rain that makes it through the tree canopy. “There was a human interest side to this part of the story,” explains Richard Boase. “The District of North Vancouver partnered with the North Shore Mentally Handicapped Association to mass produce the wooden support structures for the tree canopy climate stations. Thus, we could truly say there was broad-based North Shore community participation when you consider this in combination with what was happening in Clovelly-Caulfield.”

Paddy Sherman, Chair of the Council-appointed Clovelly-Caulfeild Neighourhood Plan Working Group, stated the following at the time: “We are impressed by the innovative and easy-to-install system that Dr. Markus Weiler of UBC and Richard Boase of North Vancouver District have developed for capturing rain that makes it through the tree canopy. The community volunteers are excited to play a part in this project. This on-the-ground research by UBC will inform the neighbourhood planning process by bringing science into the discussion of the role that trees play in the urban environment. It will also advance the state-of-the-practice in rainwater management.”


Rainfall Interception in an Urban Environment:
Research Findings

“If a tree on an urban lot is cut down, how big is the net loss on that lot?  Or if a tree is planted, how big is the benefit?  If a tree overshadows grass on one side and a rooftop on the other, how does it compare to a tree simply spreading over a lawn?  The unfortunate situation is that until now, answering these kinds of questions was largely based on what we might call informed guesswork – if they were answered at all,” states Yeganeh Asadian, the researcher whose Master’s thesis informed development of the Tree Canopy Module.

“Yet these questions are exactly the ones that need to be dealt with if we are going to properly manage this vital resource.  We need more than good principles and concepts.  To make well founded decisions, we need to be able to put numbers on the results of the decisions we make.”

“We applied a unique methodology for measuring rain/throughfall under 54 different urban trees using a system of PVC pipes hung beneath the canopy to capture the throughfall where it drained into a rain gauge attached to a data logger. To ensure that the study adequately captured the range of throughfall variability, trees were selected to sample different landscape sites (streets, parks, and natural forested areas), elevations, tree type, health condition and species, including Douglas-fir, Western red cedar, Bigleaf maple, Oak, Copper beech, Horse chestnut,  Cherry, and Poplar.”


To download a copy of the Master of Science thesis completed by Yeganeh Asadian at the University of Britsh Columbia, click on Rainfall Interception in an Urban Environment

Download a copy of the 2012 announcement by the Partnership that Rebuilt Water Balance Model Now Incorporates Tree Canopy Module.